1. Field of the Invention
This invention broadly relates to a process for producing formaldehyde from methyl mercaptans, and especially formaldehyde from methanethiol. More particularly, this invention provides a method wherein a gas stream containing a methyl mercaptan is passed in contact with a catalyst comprising certain supported metal oxides or certain bulk metal oxides in the presence of an oxidizing agent and for a time sufficient to convert at least a portion of the methyl mercaptan to formaldehyde (CH.sub.2 O) and sulfur dioxide (SO.sub.2).
2. Description of Related Art
Methyl mercaptans, such as methanethiol (CH.sub.3 SH), dimethyl sulfide (CH.sub.3 SCH.sub.3) and dimethyl disulfide (CH.sub.3 SSCH.sub.3), are found as impurities or produced as a generally undesirable by-product in a wide variety of industrial gas streams. These reduced sulfur compounds are exceedingly malodorous (detectable at parts per billion [ppb] levels), are extremely hazardous, and often are considered a pollutant. It is expected that their emission will be subject to more restrictive regulation in the future.
Noncatalytic gas phase oxidation of such reduced sulfur compounds produces primarily sulfur oxide and carbon oxide products. A. Turk et aL, Envir. Sci. Technol 23:1242-1245 (1989). Investigators have observed that oxidation in the presence of single-crystal metal surfaces (Mo, Ni, Fe, Cu) results in the formation of methane and ethane, nonselective decomposition to atomic carbon, gaseous hydrogen and the deposition of atomic sulfur on the metal surface via a stoichiometric reaction (See Wiegand et al., Surface Science, 279(1992): 105-112). Oxidation of higher mercaptans, e.g., propanethiol on oxygen-covered single-crystal metal surfaces (Rh), produced acetone via a stoichiometric reaction at low selectivity and accompanied by sulfur deposition on the metal surface (See Bol et al., J Am. Chem. Soc., 117(1995): 5351-5258). The deposition of sulfur on the metal surface obviously precludes continuous operation.
Catalysts comprising a two-dimensional metal oxide overlayer on titania and 10 silica supports, e.g., vanadia on titania, have been used for catalytically reducing nitrogen oxides (NO.sub.x) by ammonia to N.sub.2 and H.sub.2 O in the presence of sulfur oxides. Bosch et al, Catal. Today 2:369 et seq. (1988). Thus, such catalysts are known to be resistant to poisoning by sulfur oxides. It also is known that such catalysts, as well as certain bulk metal oxides catalysts, can be used to oxidize methanol to formaldehyde selectively. Busca et al, J Phys. Chem. 91:5263 et seq. (1987). Applicant now has made the discovery that these catalysts also can be used to oxidize methyl mercaptans selectively to formaldehyde in a continuous, heterogenous catalytic process without being poisoned by the reduced sulfur.